We investigate quantum effects in pattern-formation for a degenerateoptical parametric oscillator with walk-off. This device has a convective regime in which macroscopic patterns are both initiated andsustained by quantum noise. Familiar methods based on linearizationabout a pseudo-classical field fail in this regime and new approachesare required. We employ a method in which the pump field is treatedas a c-number variable but is driven by the c-number representationof the quantum sub-harmonic signal field. This allows us to include the effects of the fluctuations in the signal on the pump, which in turn actback on the signal. We find that the non-classical effects, in the form of squeezing,survive just above the threshold of the convective regime. Further above thresholdthe macroscopic quantum noise suppresses these effects.